The present invention relates to an undercoating material for photosensitive resins or, more particularly, to an undercoating material for photosensitive resins useful in the pattern formation on the surface of various kinds of substrates such as semiconductor wafers by the techniques of forming a resist film of the so-called multi-layer structure and etching as well as to a method for patterning on the surface of a substrate by the techniques of forming a resist film of the so-called multi-layer structure using a unique undercoating material followed by etching.
In the prior art manufacturing process of semiconductor integrated circuits, for example, by forming an etched pattern on the surface of a substrate such as a wafer of a silicon semiconductor, it is usual that a surface film of a photosensitive resin called a resist is first formed on the substrate surface and the resist film is irradiated with actinic rays through a photomask bearing a pattern of the desired circuit to produce a difference in the solubility of the resist in a solvent called a developer between the areas irradiated and unirradiated with the actinic rays followed by development, i.e. partial dissolution, of the resist film to form a patterned resist layer on the substrate surface with partial exposure of the unprotected substrate surface. The substrate thus provided on the surface with a patterned resist layer is then subjected to a treatment of etching followed by the removal of the resist layer. It is not rare that the cycle of the above described steps is repeated several times when the finally desired integrated circuit is relatively complicated.
In recent years, remarkable progress has been made in the techniques of the above described microlithography as an example of the fine working technology. In photolithography utilizing a process of irradiation with light for patterning, in particular, photolithography of an extremely high precision in the so-called submicron region by exposure to light is or will be no longer unimaginable in contrast to the hitherto accepted general understanding of impossibility by virtue of the improvements accomplished in connection with the accuracy in the photomask alignment, development of an instrument for exposure of the step-and-repeat type capable of giving a high resolving power, construction of the optical system and other advances although several important technical problems are left unsolved in order to achieve reproduction of the fine pattern of the photomask on the substrate with full precision despite the above mentioned several improvements in relation to the instruments undoubtedly facilitating fine working.
It is noteworthy that the flatness or degree of levelling on the surface of the substrate as a base of the semiconductor integrated circuit is not always high enough due to the unavoidable presence of several types of roughness and difference in levels. The ruggedness or difference in levels on the substrate surface is one of the major causes having adverse influences on the uniformity of thickness of the resist film provided on the surface with localized variations in the film thickness which is larger in the recessed areas than in the raised areas. Consequently, it is usual that the surface coated with a resist exhibits a wavy or undulating appearance. Such an unevenness in the thickness of the resist film directly influences the uniformity of the sensitivity characteristics of the resist film adversely affecting the results of patterning. Further, the irregularity on the substrate surface may cause diffused reflection of the projected light so that the accuracy in the dimensions of the desired pattern and the resolving power for fine patterns are greatly decreased.
As a trend in recent years, as is mentioned above, step-and-repeat type exposure instruments by projection of light called a "stepper" are increasingly introduced into the production lines of semiconductor devices by virtue of the superiority in the high resolving power replacing the conventional contact-type exposure instruments. The achievement in the steppers in respect of the high resolving power is obtained at a monochromatic wave length of light of, for example, 436 nm by the improvements in the light source or the optical system while a problem in the photolithography utilizing a monochromatic light is the generation of standing waves readily taking place, in particular, on a well-levelled substrate surface with little ruggedness.
In order to achieve further improved patterning with a high resolving power, accordingly, the above described problems must be solved by all means and it is required therefore to decrease the roughness of the substrate surface, to reduce the reflection of light on the substrate surface and, in the same time, to prevent generation of standing waves.
With an object to satisfy these requirements, proposals and attempts have been made in recent years for providing a resist film of a multilayered structure on the substrate surface. As an example of such a method of multilayered resist, a method of three-layered resist is proposed in which a substrate surface having differences in level is imparted with an increased degree of levelling by providing a relatively thick coating layer of an organic resin as an undercoating on which a thin layer of a metal or an inorganic compound formed, for example, by the CVD method and a top layer of a resist are successively provided. Although advantageous in respect of the high resolving power and high aspect ratio of the pattern formed by the method, a great disadvantage in this method is the low productivity due to the complicacy of the process to cause an extreme decrease in the throughput.
Alternatively, a method of bilayered resist is also under development in which the problem of the complicacy of the process in the above described three-layered resist can be at least partly solved without affecting the accuracy of pattern reproduction. In this method, the substrate surface is first provided with an undercoating layer of an organic resin capable of increasing the degree of levelling and reducing the reflection of light on the surface and a top coat of a resist material is provided thereon. This method, however, involves several problems and disadvantages left unsolved. For example, intermixing dissolution may sometimes take place between the undercoating and the top coat of the resist at the interface when an organic solution of the resist material is applied to the surface of the undercoating of the organic resin so that roughness is formed on the topmost surface of the bilayer consequently resulting in decreased uniformity in the thickness of the binary coating layer as a whole. In addition, the solubility behavior of the bilayer is somewhat different from the intended one in the areas where such an intermixing dissolution has taken place between the undercoating and the top coat. In such a case, at any rate, no high-fidelity reproduction of the pattern of the photomask can be obtained with a desired high precision due to the drawbacks of a decreased resolving power and incomplete development.
Accordingly, measures are taken to prevent the intermixing dissolution by subjecting the surface of the undercoating to a certain treatment prior to the application of the top coat solution thereto. A more conventional means therefor is, however, to select and use an undercoating material exhibiting a solubility behavior in a solvent quite different from that of the top coat. Exemplary of such a combination of the undercoating material and the resist material in the top coat are, for example, a combination of a rubber-based negative-working photoresist material as the undercoat and an alkali-soluble positive-working photoresist material as the top coat and a combination of a poly(methyl methacrylate) or a poly(isopropenylketone) containing a dye as the undercoating material and an alkali-soluble positive-working photoresist material as the top coat as proposed in Japanese Patent Kokai 58-51517. This method of selecting and using a combination of the undercoating and top coat quite different in the solubility behavior from each other is, however, disadvantageous in the complicacy of the process because such a bilayered resist layer requires a two-step treatment in the development after the patternwise exposure to light due to the quite different solubility behaviors of the undercoat and the top coat although this is a favorable condition to facilitate formation of the bilayered coating.